Communications system with communication terminals which are connected to a switching system via a packet-oriented communication network

ABSTRACT

A switching system is connected to a communication network via a broadband access unit having a network access interface and communication terminals are connected to the communication network via hubs. The broadband access unit has conversion units allocated to the hubs, via which conversion units a bi-directional conversion between the data format of the packet-oriented communication network and a data format within the switching system is effected. Furthermore, the broadband access unit has a switching network module for combining the data to be transmitted from the conversion units to the associated hubs.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates, generally, to a communications systemwith communication terminals connected to a switching system via apacket-oriented communication network and, more particularly, to acommunications system wherein a switching system is connected to acommunication network via a broadband access unit which has a networkaccess interface, and communication terminals are connected to thecommunication network via hubs.

2. Description of the Prior Art

Both from German Laid-Open Specification DE 196 04 244 A1, and fromSchlichthärle D.: “Hybrid ATM/ISDN Subscriber Connection to a BroadbandISPBX”, Internal Conference on Computer Communication —Proceedings ofthe Conference—Towards a New World in Computer Communication, 28 Sep.1992, XP000671919, a communications system is known in which thecommunication terminals allocated to a switching system are connected tothe switching system via an ATM-based communication network. In thisarrangement, subscriber interfaces for connecting communicationterminals are provided by a number of ATM transfer units—frequentlycalled ATM hubs—which are connected to the ATM-based connection network.The switching system and the ATM hub in each case have an ATM accessunit via which, on the one hand, a connection to the ATM-basedcommunication network is implemented and, on the other hand, abi-directional conversion between the internal data format of theswitching system or hub and the data format of the ATM-basedcommunication network is effected.

Modern ATM hubs usually have 64 subscriber interfaces for connectingcommunication terminals to an ATM-based communication network. Inparticular, ISDN (Integrated Services Digital Network) communicationterminals are connected via S₀ interfaces, or digital communicationterminals are connected via interfaces derived therefrom such as, forexample, U_(P0) interfaces, via an ATM hub to an ATM-based communicationnetwork. In general, a U_(P0) or an S₀ interface includes, on the onehand, two user data channels which, as ISDN-oriented B channels have atransmission rate of in each case 64 kBit/s and, on the other hand, asignalling channel which, as ISDN-oriented D channel, has a transmissionrate of 16 kBit/s.

The switching system and the ATM hubs are frequently connected to anATM-based communication network via a so-called STM1 (SynchronousTransfer Mode) interface having a maximum transmission bit rate of 155Mbit/s. An access module providing such an STM1 interface is used incommunications systems from the company Siemens AG under the internalname “STMA access module”. In the text which follows, this name is usedfor such access modules having an STM1 interface.

An STMA access module arranged in the switching system currentlyprovides the capability to support 32 subscriber interfaces of an ATMhub connected to an ATM-based communication network; i.e., it ispossible to carry out data transmission between the STMA access moduleand 32 different communication terminals connected to an ATM hub. Thisonly corresponds to a maximum transmission bit rate of 8 Mbit/s via theSTM1 interface provided by the STMA access module (having a maximumtransmission bit rate of 155 Mbit/s).

Thus, two STMA access modules are necessary in the switching system forsupporting all 64 subscriber interfaces of an ATM hub, which isconnected via an ATM-based communication network, with the switchingsystem. Since both the STMA access modules and the ATM access unit of anATM hub only have one STM1 interface in each case, it is necessary tointerpose an additional ATM switching module. The ATM switching moduleconcentrates the data to be transmitted between the switching system andthe ATM hub from the two STMA access modules onto the ATM access unit ofthe ATM hub or, respectively, splits these data from the ATM access unitonto the two STMA access modules.

Using an enhanced STMA access module which supports a total of 64subscriber interfaces there is no need to interpose an additionalswitching module since an STMA access module expanded in this mannersupports all 64 subscriber interfaces of an ATM hub and, thus, these canbe connected directly to the enhanced STMA access module. Thus, althoughonly one STM1 interface, provided by an enhanced STMA access module, ofthe switching system is now occupied for each ATM hub connected to theATM-based communication network, still only a maximum transmission bitrate of 16 Mbit/s is achieved via the STM1 interface (which has amaximum transmission bit rate of 155 Mbit/s).

It is an object of the present invention, therefore to specify anarrangement by which, when a number of ATM hubs are connected to apacket-oriented communication network, a data transmission can takeplace between a switching system and the ATM hubs via a network accessof the switching system utilizing the transmission bandwidth provided bythe network access.

SUMMARY OF THE INVENTION

In connection with the present invention, and to provide a betterunderstanding of the basic configuration of a switching system, ishelpful first to discuss again in greater detail principles which arealready known.

In this connection, FIG. 1, which shows a diagrammatic representation ofthe essential functional units of a switching system PBX, serves toobtain a quicker understanding of the relationships. The switchingsystem PBX has a central controller CC which can be connected to accessmodules and a switching network SN. The access modules include, inparticular, subscriber line modules SLM11 . . . SLM1 x, SLM21 . . . SLM2x, SLMn1 . . . SLMnx and so-called trunk modules TM11, TM21, TMn1.

The subscriber line modules SLM have subscriber interfaces forconnecting communication terminals KE to the switching system PBX.First, for example, ISDN communication terminals can be connected via S₀interfaces or digital communication terminals can be connected viainterfaces derived therefrom, such as, for example, U_(p0) interfaces,to the switching system PBX. Furthermore, it is possible to connectanalogue communication terminals and facsimile terminals to theswitching system PBX via analogue a/b interfaces.

The trunk modules TM11, TM21, TMn1 are used for connecting the switchingsystem PBX to communication networks or for connecting to otherswitching systems. A connection to another switching system is carriedout, for example, via so-called “PCM Highways” (Pulse Code Modulation),frequently also called primary multiplex access or S_(2M) interface inthe literature, which generally include, on the one hand, 30 user datachannels which are designed as ISDN-oriented B channels with atransmission rate of 64 kBit/s and, on the other hand, a signallingchannel which is designed as ISDN-oriented D channel with a transmissionrate of 64 kBit/s. A data transmission via such a “PCM highway” thusresults in a maximum transmission bit rate of 2 Mbit/s. A known trunkmodule TM11, TM21, TMn1 for connecting a switching system PBX to anATM-based communication network is, e.g., the “STMA access module” bythe Siemens company mentioned in the introduction to the description.

A number of peripheral modules—subscriber line modules SLM11 . . . SLM1x, SLM21 . . . SLM2 x, SLMn1 . . . SLMnx and trunk modules TM11, TM21,TMn1 can be functionally combined to form a line trunk unit LTU1, . . .,LTUn. Each line trunk unit LTU1, . . . LTUn is associated with a linetrunk unit controller LTUC1, . . . ,LTUCn which are in each caseconnected to the switching network SN and the central controller CC viaa so-called LTU link having a transmission bandwidth of 4×4 Mbit/s. Themessage exchange between the peripheral modules and the centralcontroller CC takes place via a signalling channel which is called bythe reference symbol HDLC (High Level Data Link Control) FIG. 1, in theknown HDLC point-to-multipoint method.

The line trunk units LTU1, . . . ,LTUn are also associated with aso-called signalling unit SU. This signalling unit SU suppliescommunication terminals KE connected to the switching system PBX withaudible tones and possibly with announcements stored in the signallingunit SU.

The central controller CC handles, among other things, theswitching-related processing occurring with a communication link betweencommunication terminals KE such as, e.g., the setting-up andclearing-down of the communication link. The central controller CCessentially includes a central processor DP, a processor for controllingthe signalling DCL, a clock generator PCG and a database DB.

An essential advantage of the system according to the present inventionthen consists in that a broadband access unit according to the inventionfor connecting the switching system to the packet-oriented communicationnetwork can be implemented in a simple manner in switching systemsalready existing, instead of a conventional line trunk unit, withouthaving to make any changes in the central controller of the switchingsystem.

An advantage of further embodiments of the present invention consistsin, among other things, that due to the modular configuration of thebroadband access unit, which can be equipped both with broadband accessmodules and with narrowband access modules, the broadband access unitcan be adapted in a simple manner to the current demand for broadband ornarrowband subscriber accesses or network accesses.

Thus, the broadband access unit can be operated as a separate broadbandswitching system if it is exclusively equipped with broadband accessmodules and both as a broadband switching system and, in co-operationwith the other components of the switching system, as a narrowbandswitching system if it is additionally equipped with narrowband accessmodules.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the following DetailedDescription of the Preferred Embodiments and the Drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of the functional units of a known switchingsystem;

FIG. 2 shows a structural diagram for the diagrammatic representation ofcommunication terminals connected to a switching system via apacket-oriented communication network;

FIG. 3 shows a structural diagram for the diagrammatic representation ofthe essential functional units of a broadband access unit arranged inthe switching system; and

FIG. 4 shows a structural diagram for the diagrammatic representation ofthe essential functional units of a switching and control unit of thebroadband access unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows a diagrammatic representation of a switching system PBXwith a broadband access unit BB-AE arranged instead of a conventionalline trunk unit in the switching system PBX. The broadband access unitBB-AE is connected via at least one LTU (Line Trunk Unit) links LTU-VLwith a central unit ZE, containing a switching network SN and a centralcontroller CC, of the switching system PBX. A data transmission betweenthe broadband access unit BB-AE and a central unit ZE via the LTU linkLTU-VL is effected in accordance with a time-division multiplex-orienteddata format and a data transmission with a maximum transmission bit rateof 4×4=16 Mbit/s is possible via an LTU link LTU-VL.

In addition, other line trunk units LTU1, . . . ,LTUn−1, designed asdescribed in connection with FIG. 1, are arranged in the switchingsystem PBX, the line trunk units LTU1, . . . , LTUn−1 being connected tothe central unit ZE of the switching system PBX via, in each case, oneLTU link LTU-VL. The line trunk units LTU1, . . . , LTUn−1 connectcommunication terminals to the switching system PBX via subscriberinterfaces or, respectively, implement a connection to a communicationnetwork or to another switching system. For example, communicationterminals KE are connected to the link trunk units LTU1, . . . LTUn−1.The broadband access unit BB-AE is connected via a so-called STM1(Synchronous Transfer Mode) interface, having a maximum transmission bitrate of 155 Mbit/s, to an ATM-based (Asynchronous Transfer Mode)communication network ATM-KN. Furthermore, the broadband access unitBB-AE has other interfaces for connecting communication terminals ornetworks to the broadband access unit BB-AE. For example, a UTP25interface (Unshielded Twisted Pair) having a maximum transmission bitrate of 25 Mbit/s and an Ethernet interface ES for connection to acomputer network based on the IP (Internet Protocol) is shown.

Furthermore, so-called ATM transfer units ATM-HUB—frequently called “ATMhubs” in the literature—are connected to the ATM-based communicationnetwork ATM-KN. In this arrangement, the ATM hubs ATM-HUB are connectedto the ATM-based communication network ATM-KN in each case via an accessunit AE having an STM1 interface. The ATM hubs ATM-HUB additionally havesubscriber interfaces TSS1, . . . , TSS64 for connecting communicationterminals to the ATM-oriented communication network ATM-KN. As anexample, communication terminals KE . . . , KEn are shown which areconnected to the ATM hubs ATM-HUB via the subscriber interfaces TSS11, .. . , TSS64. In particular, ISDN (Integrated Services Digital Network)communication terminals are connected via S₀ interfaces, or digitalcommunication terminals are connected via interfaces derived therefrom,such as, for example, U_(p0) interfaces, via the ATM hubs to theATM-based communication network ATM-KN. In addition, there is thepossibility of connecting analogue communication terminals to theATM-oriented communication network ATM-KN via analogue a/b interfaces.Data transmission between communication terminals KE1, . . . , KEn andthe switching system PBX is usually carried out on the basis of thetime-division multiplex-oriented data format. For transmitting databetween the communication terminals KE1, . . . , KEn and the switchingsystem PBX via the ATM-based communication network ATM-KN, abidirectional conversion is performed between the time-divisionmultiplex-oriented data format and the data format of the ATM-orientedcommunication network ATM-KN by the access units AE of the ATM hubsATM-HUB and the broadband access unit BB-AE.

FIG. 3 shows a diagrammatic representation of the essential functionalunits of the broadband access unit BB-AE. The broadband access unitBB-AE has both a broadband bus system BB-BUS and a narrowband bus systemNB-BUS for transmitting data within the access unit. In the broadbandaccess unit BB-AE, a line trunk unit controller LTUCX is arranged whichis connected both to the narrowband bus system NB-BUS and, via at leastone LTU link LTU-VL, to the central unit ZE of the switching system PBX.The line trunk unit controller LTUCX performs a bi-directionalconversion between the data format of the LTU link LTU-VL, an LTU linkLTU-VL including four time-division multiplex-oriented 4-Mbit/scommunication links, and the data format of the narrowband bus systemNB-BUS which includes a number of time-division multiplex-oriented2-Mbit/s communication links.

To convert the time-division multiplex-oriented data format in theaccess units—corresponding to the data format set up for datatransmission via the narrowband bus system NB-BUS—to the data format ofthe ATM-based communication network ATM-KN, the broadband access unitBB-AE has conversion units STMAX. The conversion units STMAX areconnected, on the one hand, via the narrowband bus system NB-BUS, ineach case via eight time-division multiplex-oriented 2-Mbit/s datacommunication links, to the line trunk unit controller LTUCX and, on theother hand, via a UTOPIA (Universal Test and Operation PHY Interface forATM) interface to the broadband bus system BB-BUS.

A bi-directional data transfer between the link trunk unit controllerLTUCX and a conversion unit STMAX is possible with a maximumtransmission bit rate of 16 Mbit/s via the eight time-divisionmultiplex-oriented 2-Mbit/s communication links. With the presenttime-division multiplex-oriented data format, this corresponds to anumber of 256 multiplex channels as a result of which a total of 64subscriber interfaces can be supported by a conversion unit STMAX.

The broadband bus system BB-BUS is connected to broadband access modulesfor connecting communication terminals or computers, or for connectingthe switching system PBX to a communication or computer network. As anexample, an STM1 access module is shown via which, for example, theswitching system PBX is connected to the ATM-based communication networkATM-KN. Furthermore, a UTP25 access module having a maximum transmissionbit rate of 25 Mbit/s for connecting computers and an Ethernet interfaceES for connecting the switching system PBX to a computer network basedon the Internet protocol is shown. As an alternative, narrowband accessmodules also can be connected to the narrowband bus system NB-BUSinstead of the broadband access modules STM1, UTP25, ES.

The access modules STM1, UTP25, ES are connected to a switching andcontrol unit CSCP (Cell Switched Central Processor) via the broadbandbus system BB-BUS and a CPU (Central Processing Unit) bus systemCPU-BUS. Overall, a total of eight modules (ES, STM1, UTP25, CSCP,LTUCX, STMAX) can be connected to the broadband bus system BB-BUS and tothe narrowband bus system NB-BUS of the broadband access unit BB-AE.

FIG. 4 shows a diagrammatic representation of the essential functionalunits of the switching and control unit CSCP. For cell-based dataswitching by the broadband access unit BB-AE, the switching and controlunit CSCP essentially has a cell-based switching matrix module BB-KN anda control unit CPU. The switching and control unit CSCP also includes 4multiplexers MUX1, . . . , MUX4 for connecting the cell-based switchingmatrix module BB-KN to the broadband bus system BB-BUS and other STM1access units STM1 for connecting the switching and control unit CSCPdirectly to the ATM-based communication network ATM-KN or to anothercommunication or computer network. For controlling a data transmission,the control unit CPU is connected to the cell-based switching matrixmodule BB-KN, to a clock generator CLK and the STM1 access units STM1via the CPU bus system CPU-BUS. To provide the switching and controlunit CSCP with a uniform clock supply, the clock generator CLK isconnected to the multiplexers MUX1, . . . , MUX4, the cell-basedswitching matrix module BB-KN and the STM1 access units STM1.

The cell-based switching matrix module BB-KN has aswitching-matrix-module-oriented memory unit SPE subdivided into twosubmemories. In the first submemory of theswitching-matrix-module-oriented memory unit SPE, a switching tableHTT—frequently called “Header Translation Table” in the literature—isstored. This header translation table HTT contains the necessaryswitching information stored in the form of a pair of values consistingof a so-called input VCI (Virtual Channel Identifier) value and aso-called output VCI value for switching ATM cells, via whichinformation an ATM cell arriving at the cell-based switching matrixmodule BB-KN is switched. The second submemory of theswitching-matrix-module-oriented memory unit SPE is used for temporarilystoring the user data transmitted in a payload area of an ATM cellduring the switching of the ATM cell in the cell-based switching matrixmodule BB-KN.

Furthermore, the cell-based switching matrix module BB-KN has twohigh-frequency UTOPIA interfaces. The cell-based switching matrix moduleBB-KN is connected via the UTOPIA interfaces to, in each case, twomultiplexers MUX1, . . . , MUX4 via in each case one 16-bit-widecell-based UTOPIA databus DB. A bi-directional data transmission rate of622 Mbit/s can be achieved via the 16-bit-wide cell-based UTOPIA databusDB. The multiplexers MUX1, . . . , MUX4, which are designed, forexample, as described in the German Patent application having theofficial reference 197 515 60.6, convert the data format of the16-bit-wide cell-based UTOPIA databus DB to the data format of the8-bit-wide broadband bus system BB-BUS. The multiplexers MUX 1, . . . ,MUX4 can be connected in each case to a maximum of four 8-bit-widedatabuses via which a maximum bidirectional data transmission rate of310 Mbit/s can be achieved.

The multiplexers MUX1, . . . , MUX4 are, thus, connected to broadbandaccess modules STM1, UTP25, ES or to conversion units STMAX either viathe broadband bus system BB-BUS or directly to the STM1 access units(shown, for example, for the multiplexer MUX4 in FIG. 4) arranged in theswitching and control unit CSCP via an 8-bit-wide UTOPIA databus.

In the text which follows, explanation is provided for the interactionof the functional units essential to data transmission between twocommunication terminals in greater detail with reference to FIGS. 1 and2.

For data transmission starting from a first communication terminal KEconnected to the ATM-based communication network ATM-KN via a subscriberinterface TSS1, . . . , TSS64 of an ATM hub ATM-HUB, to a secondcommunication terminal KE connected via a subscriber interface of a linetrunk unit LTU1, . . . , LTUn−1 of the switching system PBX, thetime-division multiplex-oriented data format usually provided for a datatransmission between the first communication terminal KE and the secondcommunication terminal KE is converted to the data format of theATM-based communication network ATM-KN in the access unit AE of the ATMhub ATM-HUB associated with the first communication terminal KE. In thisprocess, a bi-directional conversion between the time-divisionmultiplex-oriented data format and the data format of the ATM-basedcommunication network ATM-KN can be effected, for example, in accordancewith the two conversion methods proposed in the German Patentapplication having the file reference 198 436 25.4.

The converted data transmitted via the ATM-based communication networkATM-KN and received by the STM1 access module STM1 of the broadbandaccess unit BB-AE, via which the switching system PBX has been connectedto the ATM-based communication network ATM-KN, are transmitted to aconversion unit STMAX allocated to the ATM hub ATM-HUB via the broadbandbus system BB-BUS of the broadband access unit BB-AE. The conversionunit STMAX converts the received converted data back into thetime-division multiplex-oriented data format in accordance with theconversion method used in the access unit AE of the ATM hub ATM-HUBallocated to the first communication terminal KE. The data to betransmitted is then transmitted via the narrowband bus system NB-BUS tothe line trunk unit controller LTUCX which adapts the data to betransmitted for a transmission via the line trunk unit link LTU-VL(within the access unit, the data is transmitted via time-divisionmultiplex-oriented 2-Mbit/s communication links; data is transmitted viathe LTU link LTU-VL via time-division multiplex-oriented 4-Mbit/scommunication links), and then forwards it to the central unit ZE of theswitching system PBX via the LTU link LTU-VL. In the central unit ZE,the data to be transmitted is switched to the line trunk unit LTU1, . .. , LTUn−1 allocated to the second communication terminal KE by theswitching network SN of the switching system PBX, which line trunk unitforwards the data to the second communication terminal KE.

A data transmission starting from the second communication terminal KEto the first communication terminal KE analogously takes place in thereverse direction.

For a data transmission starting from the first communication terminalKE to a third communication terminal KE also connected to the ATM-basedcommunication network ATM-KN via a subscriber interface TSS1, . . . ,TSS64 of an ATM hub ATM-HUB, the time-division multiplex-oriented dataformat usually provided for a data transmission between the firstcommunication terminal KE and the third communication terminal KE isconverted to the data format of the ATM-based communication networkATM-KN in the access unit AE of the ATM hub ATM-HUB allocated to thefirst communication terminal KE.

The converted data transmitted via the ATM-based communication networkATM-KN and received by the STM1 access module STM1 of the broadbandaccess unit BB-AE are transmitted to the switching and control unit CSCPof the broadband access unit BB-AE via the broadband bus system BB-BUSof the broadband access unit BB-AE. In cases in which the switching andcontrol unit CSCP is connected directly to the ATM-based communicationnetwork ATM-KN via an STM1 interface—compared to FIG. 4—the converteddata to be transmitted can be transmitted directly to the switching andcontrol unit CSCP from the ATM hub ATM-HUB allocated to the firstcommunication terminal KE via the ATM-based communication networkATM-KN.

In the switching and control unit CSCP, the converted data to betransmitted is switched by the cell-based switching matrix module BB-KNand transmitted via the broadband bus system BB-BUS to the STM1 accessmodule STM1 via which the converted data to be transmitted is forwardedto the ATM hub ATM-HUB allocated to the third communication terminal KEvia the ATM-based communication network ATM-KN. As an alternative, theconverted data to be transmitted can be transmitted directly to therelevant ATM hub ATM-HUB via the ATM-based communication network ATM-KNfrom the switching and control unit CSCP via the STM1 interfaces of theswitching and control unit CSCP.

The access unit AE of the ATM hub ATM-HUB allocated to the thirdcommunication terminal KE converts the converted data to be transmittedback into the time-division multiplex-oriented data format in accordancewith the conversion method used in the ATM hub ATM-HUB allocated to thefirst communication terminal and is forwarded to the third communicationterminal KE.

A data transmission starting from the third communication terminal KE tothe first communication terminal KE is effected analogously in thereverse direction.

Although the present invention has been described with reference tospecific embodiments, those of skill in the art will recognize thatchanges may be made thereto without departing from the spirit and scopeof the invention as set forth in the hereafter appended claims.

1. A communications system, comprising: a packet-oriented communicationnetwork; a plurality of communication terminals, connected to thepacket-oriented communication network via a plurality of hubs; and aswitching system connected to the packet-oriented communication network,the switching system including: a broadband access unit, a central unit,a plurality of conversion units and a switching matrix module, whereinthe broadband access unit is connected to the central unit via atime-slot-oriented link and is further connected to the packet-orientedcommunication network via at least one packet-oriented network accessinterface, wherein the plurality of conversion units are allocated tothe plurality of hubs via which a bidirectional conversion between adata format of the packet-oriented communication network and atime-slot-oriented data format is effected, and wherein the switchingmatrix module combines data to be transmitted to the plurality of hubsfrom the conversion units for transmission via the packet-orientednetwork access interface.
 2. A communications system as claimed in claim1, wherein the broadband access unit has both a broadband bus system fortransmitting a packet-oriented data stream within the access unit and anarrowband bus system for transmitting a time-slot-oriented data streamwithin the access unit, and wherein the broadband bus system can becoupled to the narrowband bus system via the plurality of conversionunits.
 3. A communications system as claimed in claim 2, wherein thenarrowband bus system is connected to a line trunk unit controller viawhich the broadband access unit can be connected to the central unit,which further includes a switching network and a central controller, viathe time-slot-oriented link.
 4. A communications system as claimed inclaim 3, wherein the line trunk unit controller is connected to thecentral unit via at least one time-division multiplex-oriented 4-Mbit/sdata communication link.
 5. A communications system as claimed in claim3, wherein a number of time-division multiplex-oriented 4 Mbit/scommunication links can be determined by a number of the plurality ofconversion units arranged in the broadband access unit.
 6. Acommunications system as claimed in claim 2, wherein access unitsconnected to the narrowband bus system are connected to one another viaat least one time-division multiplex-oriented 2 Mbit/s communicationlinks.
 7. A communications system as claimed in claim 2, wherein boththe broadband bus system and the narrowband bus system have accesslocations for a plurality of access units.
 8. A communications system asclaimed in claim 6, wherein at least one of the broadband access modulesand the narrowband access modules can be connected to the accesslocations.
 9. A communications system as claimed in claim 7, wherein theaccess units can be respectively connected to the broadband bus systemvia one 15 UTOPIA.